In accordance with the state of the art, automatic transmissions intended especially for motor vehicles comprise planetary gear sets that are shifted by means of friction or switch elements, such as couplings and brakes, and are usually connected to a starter element, which is subjected to a slip effect and selectively to a starter element equipped with a bypass clutch, such as a hydrodynamic torque converter or a hydraulic coupling.
A transmission such as this can be seen in EP 0 434 525 A1. It comprises essentially an input shaft and an output shaft, which are arranged parallel to each other, a double planetary gear set arranged concentric with respect to the output shaft and five switch elements in the form of three clutches and two brakes, whose selective locking, respectively in pairs, determines the different gear ratios between the input shaft and the output shaft. For this purpose, the transmission has a front-mounted gear set and two power paths so that six forward gears are obtained by way of the selective meshing in pairs of the five switch elements.
In the first power path, two clutches for transmitting the torque from the front-mounted gear set to two elements of the double planetary gear set are required. These are arranged in the direction of the power flow essentially behind the front-mounted gear set in the direction of the double planetary gear set. In the second power path, another clutch is provided, which detachably connects the same to a further element of the double planetary gear set. The clutches are arranged herein such that the inner disk support forms the output.
A compact multi-step transmission in planetary style, intended especially for a motor vehicle, is also known from U.S. Pat. No. 6,139,463, which has two planetary gear sets and one front-mounted gear set as well as three clutches and two brakes. Two clutches C-1 and C-3 are provided in a first power path in order to transmit the torque from the front-mounted gear set to the two planetary gear sets in this known multi-step transmission. The outer disk support or the cylinder or piston and pressure equalization side of the clutch C-3 is herein connected to a first brake B-1. In addition, the inner disk support of the third clutch C-3 is connected to the cylinder or piston and pressure equalization side of the first clutch C-1, wherein the inner disk support of the first clutch C-1 is arranged on the side of the output and is connected to a sun gear of the third planetary gear set.
In addition, a multi-step transmission is known from DE 199 49 507 A1 of the Applicant, in which two forward-mounted gear sets, which are not shiftable, are provided on the input shaft and these gear sets generate two speeds on the side of the output which, aside from the speed of the input shaft, can be optionally shifted on a shiftable double planetary gear set that acts on the output shaft in such a way by means of optionally locking the employed switch elements that in order to shift from one gear into the respective next higher or lower gear only one of the two switch elements that were just actuated must be activated or deactivated.
An automatically shiftable motor vehicle transmission having three spider supported planetary sets as well as three brakes and two clutches in order to shift six forward gears and one reverse gear and having an input shaft as well as an output shaft is known furthermore from DE 199 12 480 A1. The automatically shiftable motor vehicle transmission is configured in such a way that the input shaft is connected directly to the sun gear of the second planetary gear train, and that the input shaft can be connected via the first clutch to the sun gear of the first planetary gear set and/or via the second clutch to the planet carrier of the first planetary gear set. In addition or as an alternative thereto, the sun gear of the first planetary gear train can be connected via the first brake to the housing of the transmission and/or the planet carrier of the first planetary gear set can be connected via the second brake to the housing and/or the sun gear of the third planetary gear set can be connected via the third brake to the housing.
It is the object of the present invention to propose a multi-step transmission of the kind mentioned above, in which the constructive effort is optimized and, in addition, the effectiveness in the main driving gears is improved with respect to drag loss and gearing loss. In addition, low torque should act on the switch elements and planetary gear sets and also the speeds of the shafts, switch elements and planetary gear sets should be kept as low as possible in the multi-step transmission of the invention. In addition, the number of gears as well as the transmission spread should be increased.